mi.J WHITE ON CONDITIONS OF PRESERVA.TION OF FOSSILS. 141 



sliales of the same formation only a few miles distant. Tliis may have 

 been the result of a deficiency of lime in the water in the first-named 

 instance, but it is not unlikely that the <li1Terence was produced by a 

 diminution of the substance of the shells during the process of their fos- 

 silization. 



Briefly reviewing the subject of the manner of preservation of inverte- 

 brate fossils, it appears that those of the various kinds whose original 

 composition was calcareous and apparently identical, or nearly so, have 

 resisted unequally the solvents with which they have been brought in 

 contact in the ckcumstances of their fossihzation. It thus seems that, 

 as a rule, the shells of certain families of the Conchifers have resisted 

 solution less eft'ectually than any others, and that among paleozoic fos- 

 sils the arthropomatous brachiopods, and among mesozoic fossils the 

 Ostreidae, have resisted solution most effectually. It also appears that 

 chitinous substances, although not originaUy so completely mineral-like 

 as shells and corals, are even more permanent as fossils than those which 

 were originally calcareous, since they have resisted solution more effect- 

 ually. 



JSTo explanation is at present offered of the reason why certain fossils 

 are much more perfectty preserved than others under the same environ- 

 ing circumstances ; and, indeed, it seems difficult to suggest an explana- 

 tion. As already remarked, it might a priori be expected that substances 

 so different as chitinous and calcareous shells are would differ in the 

 condition of their preservation under the same environing circumstances, 

 but it could not thus have been expected that so great a difference as 

 we have seen should exist in that respect between the calcareous sheUs 

 of the different families of mollusks. 



